This invention pertains to the art of raio devices and particularly to radio frequency circuitry including adjustable tuners.
The invention is applicable to a radio frequency transmitter for transmitting very stable high frequency transmissions and, more particularly, to a radio transmitter to be used in an extremely hazardous and inaccessible location, such as the wheel of an automobile, which will subject the circuit to extreme shock, vibration, temperature and pressure and yet maintain operation at the tuned frequency. Such a circuit may be advantageously employed in conjunction with a low tire warning system as is disclosed in U.S. Pat. No. 4,237,728. However, it will be appreciated by those skilled in the art that the invention can be readily adapted for use in other application, environments or locations as, for example, where similar radio devices are employed in other types of areas.
Since the radio circuit is to be used in such a harsh environment, and after installation will be relatively inaccessible for maintenance and retuning, it is especially desirous to have a radio circuit which is particularly sturdy and that will maintain the necessary tight frequency stability. The circuit will also involve a relatively low level power supply and accordingly only a low level power transmission may be made to a fairly closely positioned radio receiver. It is a further requirement of the invention that the circuit be dimensioned for ready attachment in a wheel rim cavity of an automobile wheel.
The various forms and types of tunable radio circuits that have been suggested for use in an automobile tire in association with a low tire warning system have met with varying degrees of success. It has been found that the defects present in most prior radio circuits and methods for tuning radio circuits are such that the devices themselves are of limited economic and practical value when subjected to the strict requirements of the present invention.
Typical prior tunable radio circuits have employed inductive coil and slug sets, adjustable capacitors, and etching and abrading methods for setting the frequency of operation of the circuit. Such methods have failed to maintain the constant tuned frequency required when employed in a radio circuit mounted in a wheel rim cavity subjected to the normal operating conditions of an automobile wheel. Other ideas to force frequency stability have included adding circuit elements which tend to stabilize frequency. However, the circuit add-ons are relatively high cost items in comparison to the radio circuit itself and have also suffered the problems of adding weight and complicating the radio circuit.
The present invention contemplates a new and improved radio frequency circuit which overcomes the bulk of the above-referred to problems and others to provide a radio circuit which is simple in design, includes and adjustable circuit element for adjusting the frequency of operation of the circuit, avoids costly frequency stabilizing add-ons and provides a high degree of frequency stablility in a radio circuit regardless of the installation environment of the radio circuit.